1. Field of the Invention
This invention releates to digital display units and more particularly, to high resolution generators for such display units and the method employed thereby.
2. Description of the Prior Art
New applications are being increasingly found for display units coupled to a data processing system. Such display units may be custom made for such purposes or may be formed of conventional commercial television sets. In either case, the information displayed is usually of the nature of characters formed of a dot matrix where the display unit employs a raster scan mode. Each horizontal line is divided into a number of discrete points or areas called picture elements (PELS). A fraction for such picture elements per line are not employed for information display but are that portion of the scan time required for a horizontal retrace and synchronization of the horizontal oscillator.
As the display screen is scanned, the dotmatrix characters are formed by character generation circuits that control the modulation of the electron beam (in the case of CRT displays), individual circuits of which are selected by character codes that are stored in a memory. This code store can be a shift register with exactly the same number of cells as there are character positions on the display screen or it may be a random access memory. In the case of a recirculating shift register, the character codes are shifted in synchronization with the raster scan thus bringing the initial character, to be displayed, back to its proper position after each complete scan.
In some display units, 30 complete scans of all of the lines making up the display are made per second. Thus, each portion of a character being displayed is on display 30 times a second for a brief period and this can cause an apparent flickering. The flickering problem is normally solved by refreshing or redrawing all of the lines in the display in two consecutive interlaced scans. A "half scan" is redrawn or refreshed in one-sixtieth of a second. Because of the 2:1 interlace between the two half scans, if a horizontal line is drawn in one half scan and is adjacent to a line drawn in the next half scan the two form a line on the display screen which does not flicker because, in essence, it is written 60 times a second. Applying this knowledge, a 6 .times. 8 dot matrix character can be displayed on a 12 .times. 16 dot matrix, by displaying each dot in the 6 .times. 8 matrix four times. This reduces the flicker considerably, as the character now seems to be written 60 times a second, instead of 30 times. However, this results in an objectionable feature in that diagonal lines have a ragged appearance since "included" corners are not provided. This ragged appearance becomes more pronounced if the characters are displayed with a finer resolution than that with which they are stored in the character generator store.
It is, then, an object of the present invention to provide an information display unit which is flicker free and in which the displayed characters do not have a ragged appearance.
It is another object of the present invention to provide a flicker free display unit for digital information which displays relatively smooth characters without regard to the resolution or size of the matrix making up the individual characters.
It is still another object of the present invention to provide high resolution character generation circuitry for a display unit that may employ a commerical television set.